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1 Department of Physics and Geology Background orce Physical Science 1421 A force is a vector quantity capable of producing motion or a change in motion. In the SI unit system, the unit of force is the Newton (N). The properties and effect of forces in the physical world is described by Newton s three laws of motion which first appeared in 1687 in Isaac Newton s book, the Principia. 1 st law of Motion (Law of Inertia) An object moving uniformly in a straight line will continue to move with constant velocity unless a net force acts on it. An object at rest remains at rest as long as no net force acts on it. 2 nd law of Motion (Law of Mass and Acceleration) The acceleration of an object is proportional to the net force acting on it and is inversely proportional to the mass of the object. Acceleration = Net orce Mass 3 rd law of Motion (Law of Action and Reaction) or every force that acts on an object, there is a reaction force that is equal in magnitude and opposite in direction. Although there are many types of forces in the Universe, they are all based on four fundamental forces. The strong and weak forces only act at very short distances and are responsible for holding nuclei together. The electromagnetic force acts between electric charges and the gravitational force acts between masses. Some other examples of forces are: resistive force acting between two objects in contact with one another is called the frictional force; and the force acting on strings called the tension. Section 1: Weight Equipment Needed Qty Equipment Needed Qty orce Sensor 1 Mass and Hanger Set 1 Balance 1 In this section use the orce Sensor to measure the weight of a hanging mass.

2 Part 1: Background Weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, and weight is a force that results from the action of gravity on matter. In many everyday situations the word "weight" continues to be used when strictly speaking "mass" is meant. or example, we say that an object "weighs one kilogram", even though the kilogram is actually a unit of mass. The SI unit of mass is the kilogram [kg] and the SI unit of weight is the Newton [N]. Weight is defined as mass times the gravitational acceleration. Weight = mass x acceleration due to gravity, where g = 9.8 m/s 2 The weight of an object changes depending on where it is measured but the mass does not change. or instance, the weight of an object will be different on the moon because the Moon s acceleration due to gravity is less than that of the Earth s. Weight is commonly measured using one of two methods. A spring scale or hydraulic or pneumatic scale measures weight force directly. If the intention is to measure mass rather than weight, then this force must be converted to mass. As explained above, this calculation depends on the strength of gravity. SAETY REMINDER ollow the directions for using the equipment. Part 2: Lab Activity PART I: Computer Setup 1. Connect the DIN plug of the orce Sensor to Analog Channel A. 2. Open the document titled orce1.ds PART II: Equipment Setup 1. Hold the orce Sensor vertically so its hook end and mass hanger is down as shown. PART III: Data Recording 1. Start data recording. Click on the Start button ( ) to start recording data. The Start button changes to a Stop button ( display. 2. Press the tare button on orce Sensor to zero the orce Sensor. ). The orce will appear in the scale 3. Add 20 grams of mass to the hanger. Make sure that the mass and the mass hanger are not swinging. 4. Read the orce value by clicking on the Stop button ( ) in DataStudio, ending data recording. p. 2

3 5. Record the force value in your data table in the lab report. 6. Calculate the weight of the mass using W = mg and record the weight in the data table in your lab report. (orce is the experimental value and weight is the accepted value.) Record your results in the Lab Report section. Section 2: Hooke s Law Equipment Needed Qty Equipment Needed Qty orce Sensor 1 Meter stick 1 Spring 1 Table Clamp 1 Part 1: Background Hooke's law is named after the physicist Robert Hooke. In physics, Hooke's law of elasticity is an approximation that states that the amount by which a material body is deformed is linearly related to the force causing the deformation. or systems that obey Hooke's law, a spring for instance, the extension produced is directly proportional to the load and is described in the following equation: Where x is the distance by which the spring is stretched (usually in meters), is the force exerted to stretch the spring (usually in Newton), and k is the spring constant or force constant of the spring. The spring constant has units of force per unit length (usually in Newton/meter). When Hook s law holds true, the graph of the orce (in the vertical axis) and the distance through which the spring is displaced (on the horizontal axis) shows a linear slope. This slope is equal to the spring constant: p. 3

4 If we pull the spring too much or past its elastic limit, the spring may snap, buckle, or permanently be deformed. SAETY REMINDER ollow the directions for using the equipment. Part 2: Lab Activity PART I: Computer Setup 1. Connect the DIN plug of the orce Sensor to Analog Channel A. 2. Open the document titled orce2.ds PART II: Equipment Setup 1. Slip the post of the Universal Table Clamp through one of the end-loops of the spring. 2. Screw down the clamp to the edge of a table. (See photo) 3. Line up the 0 cm mark of the Meter stick to the end of the spring. (See photo.) Note: You may choose to define the "end of the spring" as either the end of the end-hoop or the end of the coils. However, you must make all measurements from this position once it has been determined. 4. "Zero" the orce Sensor by holding it horizontally on the table and pressing the "Tare" button. 5. Attach the hook of the orce Sensor to the other end-loop of the spring. PART III: Data Recording 1. Press the Start button ( ) in DataStudio to start recording data. 2. Pull the orce Sensor until the spring is stretched 5 cm. 3. Press the Keep button ( ) in DataStudio to save the current measurement. 4. When prompted, enter the amount the spring has been stretched then click OK. 5. Repeat Steps 2-4 for 10 cm, 15 cm, 20 cm, 25 cm and 30 cm. Do not overstretch the spring. 6. Press the Stop button ( ) in DataStudio. Analyzing the Data 1. rom the "it" ( ) menu button select "Linear it." 2. Record the slope and vertical intercept in the lab report section and print the graph p. 4

5 Record your results in the Lab Report section. Section 3: riction Equipment Needed Qty Equipment Needed Qty riction Tray 1 orce Sensor 1 500g Mass Bars 4 String 20 cm Part 1: Background riction is the force that opposes motion or the tendency toward motion of two surfaces in contact. riction between solid objects and fluids (gases or liquids) is called drag. The direction of the frictional force is always opposite to the direction of the motion. The coefficient of friction is the ratio of the force of friction between two bodies to the force pressing the two bodies together. The coefficient of friction depends on the materials used -- for example, ice on steel has a low coefficient of friction (the two materials slide past each other easily), while rubber on pavement has a high coefficient of friction (the materials do not slide past each other easily). There are two types of friction: The kinetic friction and the static friction. Static friction exists between two objects that are not moving relative to each other (like a book on a desk). When a force is applied to an object resting on a surface, it will not move until the force applied to it is greater than the maximum force due to static friction. The coefficient of static friction, denoted as s is simply the ratio between the maximum static frictional force ( s ) and the normal force ( N ): s s N Kinetic friction exists between two surfaces that are in relative motion with respect to each other. To keep an object moving at a constant velocity, a force must be applied to the object equal to the kinetic frictional force. Hence, the coefficient of kinetic friction ( k ) is the ratio between the kinetic frictional force ( k ) and the normal force ( N ): k The static friction is in most cases higher than the kinetic friction. k N or this lab we are going to find the N by multiplying the mass of the object and acceleration due to gravity ( N = m X g ) SAETY REMINDER ollow the directions for using the equipment. p. 5

6 Part 2: Lab Activity PART I: Computer Setup 1. Connect the DIN plug of the orce Sensor to Analog Channel A. 2. Open the document titled orce3.ds PART II: Equipment Setup 1. Cut about 20 cm of string. 2. Make a loop on each side of the string and connect the riction Tray to the orce Sensor as shown in the figure. PART III: Data Recording 1. With no tension on the string, press the "tare" or "zero" button on the force sensor. 2. Place one mass in the riction Tray. 3. Press the Start button ( ) in DataStudio to start recording data. DataStudio will automatically stop collecting data after 5.0 seconds. 4. Place the riction Tray on a surface and hold the orce Sensor. With the force sensor tied to the tray, slowly pull the riction Tray horizontally, from rest, across the lab table until it reaches a constant velocity. Continue pulling at a constant velocity until the computer stops recording. 5. Measure the weight of the riction Tray with the bar mass. Weight of the riction Tray is the normal force ( N ). 6. Use the cursor to highlight the region that corresponds to the maximum static friction. The legend box gives the value of the maximum Static riction. Enter this value into the corresponding STATIC data table. Also, enter the value of the riction Tray's normal force. 7. Use the cursor to highlight the region where the velocity is constant. The legend box gives the MEAN or average Kinetic riction. Enter this value into the corresponding KINETIC data table. Also, enter the value of the riction Tray's normal force. 8. Repeat steps 3-7 with two masses on the riction Tray. 9. Repeat steps 3-7 with three masses on the riction Tray. 10. Repeat steps 3-7 with four masses on the riction Tray. p. 6

7 Analyzing the Data 1. Click on the graph labeled "riction vs. Normal." 2. In the legend box, highlight the Data that corresponds to the static frictional force. rom the it ( intercept. ) menu at the top of the graph, select "Linear it". Record the slope and vertical 3. rom the legend box, highlight the Data that corresponds to the kinetic frictional force. rom the it menu at the top of the graph, select "Linear it". Record the slope and vertical intercept. Record your results in the Lab Report section. p. 7

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